The present invention relates to a light exposure technique used to transfer a pattern into an LSI and the like, in particular, a technique which is effectively applied to a light exposure method and a light exposure apparatus in which extreme ultraviolet (hereinafter referred to as EUV) rays are used as exposure light.
An LSI is produced by a lithographic technique of radiating exposure light onto a mask that is an original plate in which a circuit pattern is drawn, which may be referred to as a reticle, thereby transferring the circuit pattern onto a surface of a semiconductor wafer (hereinafter referred to as a wafer) by aid of a demagnification optical system.
As LSIs have been made higher in integration degree and action-speed in recent years, a tendency that their circuit patterns are made minuter has been rapidly enhanced. The method used for making the circuit patterns minuter is generally a method of making the wavelengths of exposure light shorter. Specifically, the method for making the circuit patterns minuter has been transitioning from lithographic techniques using, as exposure light, ultraviolet rays, such as the g-ray (wavelength: 436 nm) or the i-ray (wavelength: 365 nm), to lithographic techniques using, as exposure light, the KrF excimer laser (wavelength: 248 nm) or the ArF excimer laser (wavelength: 193 nm). Recently, in order to make the pattern even minuter, immersion ArF lithography, in which the refractive index of water is used, or a double patterning technique of performing light exposure two times is also being applied to mass production of LSIs.
Furthermore, researches have been made recently about a lithographic technique using, as exposure light, an EUV ray (wavelength: 13.5 nm) as a technique using a high-energy beam having a shorter wavelength. When the EUV ray is used as exposure light, the size of circuit patterns that can be resolved becomes 1/10 or less of the wavelength of ArF. Thus, attention has been paid to this technique as a method for forming extremely minute patterns.
When the EUV ray is used, a mask therefor is of a reflection type. A lightening optical system and a projection optical system therefor are also composed of reflection-type members, that is, mirrors. An EUV exposure apparatus is composed of a light source for emitting an exposure light-bundle, a lightening optical system for lightening a mask, which is an original plate, with the exposure light-bundle, a projection optical system for projecting the pattern of the mask to an exposure-receiving object (i.e., an object which is to be exposed to the light-bundle), a stage on which the mask is to be put, a stage on which the exposure-receiving object is to be put, a space for holding the projection optical system, and others. The exposure-receiving object is a wafer having a surface onto which a photosensitive material called a resist is painted.
In general, EUV rays are absorbed into all materials, so that the air cannot transmit the EUV rays. About light exposure apparatuses using EUV rays, therefore, in order to cause exposure light to reach onto a surface of a wafer while the light has a sufficient illuminance, it is necessary to decrease or exclude any light-absorbing material in a path for the exposure light, thereby keeping the optical path space (concerned) into a high vacuum state. It is also necessary that the optical path space is filled with a material from which an emission gas is discharged as slightly as possible.
In the above-mentioned exposure apparatus, in which an EUV ray is used as exposure light, the following remain in its optical path space: any emission gas generated from the resist (concerned) by irradiation with the EUV ray, and any gas generated from substances present in the exposure apparatus. Herein, the emission gas denotes a gas that is generated by the decomposition of the composition of the resist when the resist is exposed to the EUV ray, and is made mainly of a carbon compound. When this emission gas is excited by the EUV ray, molecules of the carbon compound are bonded to each other to turn into deposits called the so-called contaminations. The deposits adhere onto one or more surfaces of the mask or the optical systems (mirrors).
When the contaminations adhere onto the surface(s) of the mask or the mirrors, the reflectivity of the mirror(s) lowers so that the light quantity of the EUV ray reaching to the wafer surface (concerned) is reduced. As a result, a light exposure period is increased which is necessary for transferring the circuit pattern of the mask to the resist. Moreover, the illuminance of the EUV ray becomes largely uneven, and the wave front aberration is also increased. For these and other reasons, optical performances of the EUV exposure apparatus are remarkably deteriorated so that the precision of the transfer of the circuit pattern is also declined.
Thus, as disclosed in Patent Documents 1 to 3 listed up below and others, suggested are various techniques for removing contaminations generated in an EUV apparatus.
Patent Document 1 (Japanese Unexamined Patent Publication No. 2004-356410) discloses a technique of setting electrodes for collecting contaminations or a device for ionizing contaminations around an opening for EUV ray transmission, thereby restraining the contaminations from adhering onto surfaces of mirrors and others.
Patent Document 2 (Japanese Unexamined Patent Publication No. 2006-269942) discloses a technique of setting up an inert-gas-supplying device in an optical path space in an EUV exposure apparatus and further making a gas-discharging space between the optical path space and a wafer stage space, thereby discharging contaminations generated from the resist (concerned), together with inert gas therefrom.
Patent Document 3 (Japanese Unexamined Patent Publication No. 2005-101537) discloses a technique of setting a cold trap, such as a cryopanel, in an optical path space in an EUV exposure apparatus, thereby adsorbing contaminations.